1. Field of the Invention
The present invention relates to a method of producing a solar battery and, more particularly, to a solar battery capable of improving conversion efficiency by reducing received light loss and power transmission loss, and a method of producing the same.
2. Description of the Related Art
FIGS. 12A-12E are schematic sectional views of producing processes showing an example of a method of producing a solar battery widely used in solar power generation systems for residential buildings. First, a p-type silicon substrate 101 is wet-etched on the surface thereof by using a solution of potassium hydroxide or sodium hydroxide with a concentration in a range from 10% to 50%, thereby forming a texture. Then phosphorus is diffused in the surface of the p-type silicon substrate 101 thereby to form an n-type impurity diffused layer 102 over the entire surface of the p-type silicon substrate 101, and further an anti-reflective layer 103 made of silicon nitride is formed on the surface of the n-type impurity diffused layer 102 (FIG. 12A). Then the front surface of the anti-reflective layer 103 is coated with a silver paste by, for example, screen-printing, thereby to form a comb-shaped silver paste electrode 104xe2x80x2 (FIG. 12B). Next, the back surface of the p-type semiconductor substrate 101 is coated with an aluminum paste by, for example, screen-printing thereby to form an aluminum paste electrode 105xe2x80x2 (FIG. 12C). The silver paste electrode 104xe2x80x2 and the aluminum paste electrode 105xe2x80x2 are sintered by firing at a temperature not lower than 600xc2x0 C., thereby to form a front electrode 104 and a back electrode 105 and complete a solar battery (FIG. 12D).
During the sintering process, silicon nitride contained in the anti-reflective layer 103 is molten and the silver paste electrode 104xe2x80x2 penetrates through the anti-reflective layer 103 to reach the n-type impurity diffused layer 102, thereby establishing electrical continuity between the front electrode 104 and the n-type impurity diffusion layer 102. This technique is called fire-through process and is disclosed in, for example, Japanese Laid-open Patent Publication No. 10-233581.
FIG. 13 is a schematic perspective view showing the structure of a solar battery produced by the method described above. A plurality of front electrodes 104 having a ridge shape are disposed at predetermined spaces from each other on the surface of the n-type impurity diffused layer 102, and the anti-reflective layers 103 are disposed on both sides of the ridge-shaped front electrodes 104. The front electrode formed by the screen-printing process has a cross section of, for example, mountain-like shape 55 xcexcm in width and about 15 xcexcm in height (J. Nijs et al., First World Conference on Photovoltaic Energy Conversion, 1994, p.1242). The operation of the solar battery will be described below with reference to FIG. 14. Incident light 106 that has passed through the anti-reflective layer 103 and reached the p-type semiconductor substrate 101 generates a current 107. The current 107 is collected through the n-type impurity diffused layer 102 to the front electrode 104, and is taken out of the solar battery through the front electrode 104. In the drawing, detail of the texture is omitted in order to make the structure of the front electrode easier to comprehend.
As shown in FIG. 14, the front electrode 104 is opaque and therefore reflects the incident light 106 on the surface thereof, resulting in reduced light reception area and a loss in received light. Also because the front electrode 104 has an electrical resistance that is inversely proportional to the sectional area thereof, the electrode causes a transmission loss when taking out the current 107 to the outside. Consequently, it is necessary to decrease the width of the front electrode and increase the cross sectional area of the front electrode in order to reduce the received light loss and the transmission loss. For this purpose, the cross-section of the front electrode is preferably small in width and large in height.
However, there has been such a problem that, even when a silver paste electrode having a small width is formed by the screen-printing process or the like on the surface of the anti-reflective layer, the resultant front electrode that is formed has a flat cross section that is large in width. When the ratio of height to width is used as an index of sectional profile of the electrode, the electrode in the case of J. Nijs et al. quoted above has the ratio of height to width as small as 0.27, and it has been difficult to achieve a ratio greater than this value.
The present inventors have found that the cause of the front electrode being formed only in a flat cross section that is large in width is that the silver paste electrode spreads in the crosswise direction after printing. FIG. 15 is a schematic diagram showing the shape of the silver paste electrode, that was formed by the screen-printing process or the like on the surface of the anti-reflective layer, changing with time. The silver paste electrode 108xe2x80x2 immediately after being printed spreads with time along the surface of the anti-reflective layer 103 in the crosswise direction by gravity, thereby turning to the flat silver paste electrode 104xe2x80x2. Although viscosity of the silver paste may be increased in order to restrict the lateral spreading, the increased viscosity increases the time taken to print the silver paste electrode, thus making it difficult to decrease the production cost.
An object of the present invention is to solve the problems described above and provide a solar battery reducing receiver light loss and power transmission loss, and a method of producing the same by controlling the sectional shape of the front electrode.
In order to achieve the object described above, the method of producing a solar battery of the present invention comprises the steps of stacking a semiconductor layer of second conductivity type and an anti-reflective layer on the surface of a semiconductor substrate of first conductivity type, forming a coated-film electrode by applying a coating solution containing an electrode material on the anti-reflective layer, and forming a front electrode that penetrates the anti-reflective layer and electrically connects with the semiconductor layer by firing the coated-film electrode, wherein a water-repellent layer is formed on the anti-reflective layer prior to the formation of the coated-film electrode, the coated-film electrode having a desired fine line is formed by printing the coating solution on the water-repellent layer, so that the coating solution is suppressed from spreading over the surface of the water-repellent layer, and the coated-film electrode of narrow ridge shape is thereby formed.
According to the method of the present invention, since the water-repellent layer is formed on the light receiving surface prior to the formation of the coated-film electrode, and the coated-film electrode is formed on the surface of the water-repellent layer by printing, the coating solution can be prevented from spreading along the surface of the water-repellent layer, thereby making it possible to form the coated-film electrode having small width. Specifically, although the coating solution is forced to spread along the surface of the water-repellent layer by the gravity during printing, the water-repellent layer is difficult to wet with the coating solution and the coating solution contracts to minimize the surface area thereof. Thus the narrow coated-film electrode is formed. The water-repellent layer in the present invention refers to a layer having not only water repellency that is difficult to wet with water solution but also oil repellency that is difficult to wet with an organic solvent. Consequently, a coating solution containing an electrode material and an organic solvent added as a dispersion medium also does not easily wet the water-repellent layer of the present invention.
Also according to the method of the present invention, the water-repellent layer may be removed from portion where the coated-film electrodes is to be formed, after the water-repellent layer has been formed. The portion where the water-repellent layer is removed becomes slots. When the coating solution is applied in the slots, steps in the slots function as a barrier to restrict the movement of the paste in the crosswise direction. This makes it possible to reduce the width and increase the height of the coated-film electrode.
Also according to the method of the present invention, the water-repellent layer may be removed except for the portion that would be right below the coated-film electrode, after forming the water-repellent layer. In case the water-repellent layer absorbs the incident light, removing the water-repellent layer reduces the light reception loss caused by the water-repellent layer.
Also according to the method of the present invention, the water-repellent layer may contain a fluorine-containing surfactant as a water-repellent material. High water repellency can be achieved with a small amount of the fluorine-containing surfactant.
Also according to the method of the present invention, the water-repellent layer may be removed through thermal decomposition in the step of firing the coated-film electrode. In case the water-repellent layer absorbs the incident light, removing the water-repellent layer reduces the light reception loss caused by the water-repellent layer.
The solar battery of the present invention comprises, a semiconductor substrate of first conductivity type, a semiconductor layer of second conductivity type and an anti-reflective layer that are stacked on the surface of the semiconductor substrate of first conductivity type, and a front electrode of narrow ridge shape that penetrates the anti-reflective layer and electrically connects with the semiconductor layer, wherein the ratio of height to width in a cross-section of a crosswise direction of the front electrode is 0.4 or greater. Since the front electrode can be made higher than that of the prior art, the area that is capable of receiving the incident light can be increased, thus making it possible to reduce the light reception loss and improve the conversion efficiency of the solar battery. Also because the cross sectional area of the front electrode can be increased, power transmission loss of current can also be reduced.
The ratio of height to width can be made greater than 0.6, thereby reducing the light reception loss and the power transmission loss further.
In the solar battery of the present invention, width of the front electrode is in a range from 30 to 120 xcexcm, more preferably in a range from 30 to 70 xcexcm.
The solar battery of the present invention may also include the water-repellent layers that are formed on the anti-reflective layer and interpose the ridge-shaped front electrodes. The water-repellent layers formed on the surface of the anti-reflective layer have the effect of restricting water from depositing thereon and preventing the anti-reflective layer from being stained.
In the solar battery of the present invention, the water-repellent layer may contain a fluorine-containing surfactant. Since the fluorine-containing surfactant is contained as the water-repellent material, the water-repellent layer has high water repellency and improves the effect of preventing the anti-reflective layer from being stained further.